Isotropic coke has a thermal expansion approximately equal along the three major crystalline axes. This thermal expansion is normally expressed as CTE (i.e., coefficient of thermal expansion) over a given temperature range such as 30.degree.-530.degree. C. or 30.degree.-100.degree. C. Isotropic coke is also indicated by a CTE ratio, which is the ratio of radial CTE divided by axial CTE measured on a graphitized extruded rod. Acceptable isotropic coke has a CTE ratio of less than about 1.5 or a CTE ratio in the range of about 1.0-1.5.
Isotropic coke is used to produce hexagonal graphite logs which serve as moderators in high temperature gascooled nuclear reactors. This type of coke has been produced in the past from natural products such as gilsonite. The production of such graphite logs from gilsonite and the use thereof are described in U.S. Pat. Nos. such as 3,231,521 to Sturges, 3,245,880 to Martin et al., and 3,321,375 to Martin et al. U.S. Pat. No. 3,112,181 to Peterson et al., describes the production of isotropic coke using petroleum distillates. Contaminants, such as boron, vanadium, and sulfur, have prohibited the use of some materials as the source of isotropic coke suitable for use in nuclear reactors. Less than about 1.6 weight percent sulfur is preferred to avoid puffing problems upon graphitization and fabrication of the coke. The supply of isotropic coke has been limited by availability of source materials, such as gilsonite and expensive petroleum distillates.
U.S. Pat. No. 3,960,704 describes a process in which a low sulfur residuum, such as bottoms from the fractionation of virgin feedstocks, is air-blown to increase its softening point. The air-blown resid is then subjected to delayed coking to produce isotropic coke having a CTE ratio less than 1.5.
It would be desirable to provide a process for the conversion of low sulfur residual oils to isotropic coke without first air-blowing the residual oils.